1. Field of the Invention
The present invention relates to a catalyst converter that is housed in and fixed to piping for constituting an exhaust system for exhaust gas.
2. Description of Related Art
Various efforts have been made in various industries worldwide for reduction in environmental impact loads, and particularly, the automotive industry has strived for promotion of so-called eco-friendly cars such as hybrid electric vehicles and electric vehicles as well as gasoline vehicles with superior fuel economy performance and development to aim at further improved performance thereof.
In general, a catalyst converter for purifying exhaust gas is disposed in an exhaust system for exhaust gas that connects a vehicle engine and a muffler.
An engine emits environmentally harmful substances such as CO and NOx as well as unburned HC and VOC, and thus, in order to convert such harmful substances to permissible substances, a catalyst layer that is formed of a noble metal catalyst such as palladium or platinum is formed in a cell wall of a substrate having a number of cells. More specifically, in the cell walls of a number of the cells, the catalyst layer is formed in a longitudinal direction of the substrate that is also a direction in which the exhaust gas flows. When the exhaust gas passes through the catalyst converter having the thus-configured substrate, CO is converted to CO2, NOx is converted to N2 and O2, and VOC is burned to produce CO2 and H2O.
For example, as for the catalyst converter, the cell density of a substrate having cells with a honeycomb structure is generally uniform. However, because flow rate distribution of the exhaust gas in a center area of a cross section of the substrate is higher than that in a peripheral area, this raises a problem of insufficient use of the catalyst layer in the entire substrate. In consideration of such flow rate distribution of the exhaust gas, the catalyst converter is adapted to have the higher cell density in the center area than in the peripheral area of the substrate. Accordingly, it is possible to reduce a difference in the flow rate distribution within the cross section of the substrate as much as possible and thus is possible to purify the exhaust gas by effectively using the catalyst layer of the entire catalyst converter.
Japanese Patent Application Publication No. 2002-177794 (JP 2002-177794 A) discloses a technique to improve exhaust gas purification performance by varying an amount of the noble metal catalyst deposited in a center area (center section herein) from that deposited in a peripheral area (outer peripheral section herein) of a substrate that has a honeycomb structure and whose cell density is uniform in the entire catalyst converter (here, catalyst body). More specifically, of a catalyst body, the amount of the catalyst per unit volume that is deposited in the center section with a large gas flow is set 1.1 times or more of that in the outer peripheral section. However, problems such that it is difficult to expect the high exhaust gas purification performance due to the uniform cell density in the entire catalyst and that a large quantity of hydrogen sulfide that can be a cause of a bad smell is produced in a process of purifying the exhaust gas by increasing the amount of the noble metal catalyst are inherent to this technique.
In addition, Japanese Patent Application Publication No. 2006-281134 (JP 2006-281134 A) discloses a honeycomb structure in which the cell density is reduced from a center area to an outer peripheral area. Accordingly, by varying the cell density within a cross section of the honeycomb structure, it is possible to adjust the flow rate distribution of the exhaust gas to be as uniform as possible within the cross section of the honeycomb structure. However, it has been identified that the simple adjustment of the flow rate distribution to be as uniform as possible by adjusting the cell density just as described does not lead to the efficient use of the entire catalyst layer forming the honeycomb structure during purification of the exhaust gas.